test: enhance error accumulator and form builder tests, add marshaller tests

[Whitea029]

Describe the change
enhance error accumulator and form builder tests, add marshaller tests

Describe your solution
added some edge case tests to improve test coverage

Issue: #997

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.48%. Comparing base (4d2e7ab) to head (ebfffde).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #999      +/-   ##
==========================================
+ Coverage   85.93%   86.48%   +0.54%     
==========================================
  Files          43       43              
  Lines        2268     2271       +3     
==========================================
+ Hits         1949     1964      +15     
+ Misses        300      287      -13     
- Partials       19       20       +1     

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[Copilot]

Pull Request Overview

This PR enhances test coverage for the JSON marshaller/unmarshaler, error accumulator, and form builder modules by introducing additional edge case tests and refining error handling.

• Adds tests for valid and invalid JSON (un)marshalling
• Introduces extensive tests for form builder functionality including file uploads, special characters, and error propagation
• Refactors error accumulator tests to verify multiple write scenarios

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/unmarshaler_test.go	New tests for JSON unmarshalling including normal, invalid, and nil inputs.
internal/marshaller_test.go	New tests for JSON marshalling with normal, invalid, and empty values.
internal/form_builder_test.go	Additional tests for form builder behavior in various scenarios.
internal/form_builder.go	Adds an error check in WriteField to disallow an empty fieldname.
internal/error_accumulator_test.go	Refactored tests to verify error accumulation behavior.

Comments suppressed due to low confidence (1)

internal/form_builder_test.go:44

• The code references errors.New but does not import the 'errors' package, which may result in a compilation failure. Please add 'import "errors"' to the import block.

errorMock := errors.New("mock close error")

[Copilot]

Pull Request Overview

This PR enhances test coverage for error accumulation, form building, and marshalling behavior by adding new test cases and edge case verifications.

• Added tests for JSON unmarshalling/marshalling functionality covering normal, invalid, and empty input scenarios.
• Extended form builder tests to include multiple file uploads, special characters, concurrent file handling, and field edge cases.
• Updated error accumulator tests to validate sequential error writes and buffer initialization.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
internal/unmarshaler_test.go	New tests for verifying correct and erroneous JSON unmarshalling behavior.
internal/marshaller_test.go	New tests for validating JSON marshalling, including handling of unsupported types & nil.
internal/form_builder_test.go	Extended tests covering file uploads, field writing errors, and special filename handling.
internal/form_builder.go	Added a check to ensure field names are not empty in WriteField.
internal/error_accumulator_test.go	Updated tests to verify multiple error writes and correct initial buffer state.

Comments suppressed due to low confidence (1)

internal/error_accumulator_test.go:18

• [nitpick] Ensure that the concatenation order of error messages in the accumulator is guaranteed by design; if not, consider adding a delimiter or additional checks in the test.

expected := "{\"error\": \"test1\"}{\"error\": \"test2\"}"

[Copilot]

Pull Request Overview

This PR enhances test coverage by adding new tests for the error accumulator, form builder, and marshaller, and by improving existing tests for JSON unmarshaling.

• Added tests for normal, invalid, and edge case scenarios for JSON unmarshaler and marshaller.
• Extended form builder tests to cover file uploads, content type validation, and error propagation.
• Updated error accumulator tests for improved error concatenation validation.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
internal/unmarshaler_test.go	Tests verifying correct JSON unmarshaling behavior in various cases.
internal/marshaller_test.go	Tests validating JSON marshalling including handling of invalid inputs.
internal/form_builder_test.go	New tests covering form builder behaviors like file uploads and error handling.
internal/form_builder.go	Added fieldname validation to prevent empty field names.
internal/error_accumulator_test.go	Updated tests for error accumulation and concatenation behavior.

[Copilot]

[nitpick] Consider initializing a new FormBuilder within each subtest in TestMultiPartFormUploads to avoid potential side-effects from sharing mutable state across subtests.

Suggested change

	body := &bytes.Buffer{}
	builder := NewFormBuilder(body)
	t.Run("MultipleFiles", func(t *testing.T) {
	t.Run("MultipleFiles", func(t *testing.T) {
	body := &bytes.Buffer{}
	builder := NewFormBuilder(body)